When you click on an opposing faction fire, you get a quest box that pops up (you are not flagged for pvp)
While the quest box is up on your screen, mount up on your flying mount, then click complete and fly away. As soon as you click Complete, it will flag you for PVP.
If you need to get rid of the PVP flag for the next fire, just zone into an instance and zone back out. (I also use this in Hellfire at the towers)
[Edited by Khorium for spelling and grammar.]